If your grandma used to warn you about the risks of running around in the middle of the summer, she wasn’t wrong.
Extreme temperatures increase the stress on your body many-fold. Blood that might normally be used to power your muscles is redirected to your skin to make sweat that’ll help you thermoregulate. As your sweat rate increases, so does the risk for significant dehydration—cognitive performance can decline after a 2 percent drop in bodyweight.
Your heart rate rises accordingly, trying to move blood to the many places it’s needed. Humidity amplifies this effect, reducing the cooling effect of sweat as it pools on your skin instead of evaporating. You feel thirsty, strained and hot. The end result is that a given pace is more difficult to maintain. And if you’re racing, good luck: the faster the pace, the higher the magnitude of these effects.
The body, however, is clever. Expose it to a given stimulus enough and it will adapt out of necessity. Joshua Guy and his colleagues published an article in the journal Sports Medicine (March 2015) looking at this very topic and found that after 8 to 14 days of heat exposure athletes exercised with lower heart rates than before at the same speed, sweated earlier and had lower core temperatures. These adaptations to heat training made the athletes more efficient in all conditions.